In telephone systems it is customary to connect the voice-receiving and voice-transmitting devices of a subscriber station, such as an earpiece and a microphone, in mutually conjugate relationship to the associated line with the aid of a coupling network of the inductive type, known as a hybrid coil. A disadvantage of such inductive couplers is their limited ability to suppress the so-called local effect, i.e. the echoing of a speaker's voice into his own receiver. Another drawback is the need for using high-resistance transmitting devices, such as carbon microphones, which are sensitive to ambient temperatures and tend to deteriorate with age. The need for providing and servicing different types of electro-acoustic transducers for reception and transmission is a further inconvenience.
It is also known in telephone systems to couple two mutually conjugate two-wire lines with a common two-wire line, either at a subscriber station or at a central office, via a predominantly resistive bridge circuit having the two conjugate lines connected across respective diagonals thereof, one of the four impedance arms of the bridge being constituted by the resistance of the common line which the other three arms consist of fixed resistors. If the line resistance does not vary significantly, the three fixed resistors can be readily dimensioned to balance the bridge for proper decoupling of the two conjugate lines (i.e. the local transmitting and receiving circuits) from each other. Since, however, the resistance of a subscriber line depends upon its effective length, that parameter is subject to change, e.g. upon extension of the line of a calling subscriber to a called subscriber by the central office.